In measuring the absorbancy of a specimen with a conventional chromatoscanner, a fixed light beam is applied to a specimen under measurement and the stage supporting the specimen is moved in the X direction and in the Y direction so that the light beam scans the specimen in a zigzag manner. However, the method of moving the stage while maintaining the specimen in a fixed position is disadvantageous because the large inertial force of the stage limits, to a certain degree, the ability to increase scanning speed. Examples of devices of the prior art include U.S. Pat. Nos. 3,994,587, 4,013,364, 4,145,139 and 4,150,899.
In order to eliminate the above-described drawback, a so-called "flying spot type" chromatoscanner has been proposed in which the light beam applied to the specimen is moved. In the chromatoscanner, in order to scan the specimen with the light beam it is necessary to swing a light beam focusing mirror about a rotary shaft or to swing a slit whose configuration is similar to the section of the light beam to be focused on the specimen. Where it is required to provide a high-speed chromatoscanner, however, the method of swinging the mirror (or rotating the mirror in one direction) is unacceptable in that the inertia of the mirror determines the upper limit of the scanning speed. The method of swinging the slit is disadvantageous in that, since it is required to rotate the motor (which is generally a step motor) frequently and alternately in the forward direction and the reverse direction, it is impossible to rotate the motor at high speed.